In stick or MIG welding, pulsed arc welding is a relatively new art. It is intended to obtain, by pulsing, many of the advantages of spray transfer welding or similar high current welding techniques and at the same time some of the benefits of globular transfer welding or similar operation at low current levels. By operating intermittently at the spray transfer current level and then at the globular current level, it is possible to perform a pulsed spray process of welding at current levels much below those required for continuous spray transfer but still avoid the poorer quality welds which characterize globular welding.
The need for improved pulsed arc welding has been apparent as past apparatus has been erratic and unprecise. If one attempts to weld parts which do not have adequate heat transfer characteristics, the need to minimize heat input but attain spray levels of voltage and current become apparent to avoid overheating and excess penetration. For example, when vertical welding or overhead welding is performed, the high current necessary for spray transfer will result in a molten pool which can not be retained without a high level of thermal conductivity in the workpiece and thus, the formation of a satisfactory weld joint often becomes impossible. In the case of thin material, the high levels of weld current necessary for spray transfer result in burnthrough of the workpieces.
There have been efforts to cope with these problems in MIG welding including the use of smaller diameter electrodes and arranging work in the flat position rather than the vertical, horizontal and overhead positions. However, such expedients are obviously not practicable in many cases.
Thus, the pulsed spray transfer process was developed for MIG welding to switch the welding current back and forth between the spray transfer level of current and the globular transfer level. By this technique, there is an opportunity for cooling while the globular transfer is beginning. However, before the globular material is released the current increases to the spray transfer level and thus, accomplishes expedited spray transfer welding and metal deposition. This technique was known heretofore and is described in part at Pages 97-103 of the text "Welding Technology" published in 1968 by the American Technical Society, Chicago, Ill. 60637. Similar pulsed techniques have been tried for TIG, stick and plasma welding also.
Heretofore, the equipment utilized for pulsed arc welding has attempted to control the weld current levels exclusively through the use of a saturable reactor in the secondary winding of the weld transformer. Such control of this saturable reactor was typically by a DC control winding manually adjusted to attempt to increase the reactance of the saturable reactor during an interval for low current operation and then add DC current for an interval of high current operation during which the saturable reactor was more highly saturated. Such techniques have proven to be relatively slow, inaccurate, unpredictable and unsatisfactory. Furthermore, the time intervals for high and low current operation have also been crudely controlled on some occasions with the use of a relaxation or similar oscillator which drives a pulse width switch which in turn is fed to a driver for a reactor control device. Such techniques for time control have been relatively inaccurate and unstable and have limited the range of precision control.